PROJECT SUMMARY/ABSTRACT This application is a request for funding of a research program focused on understanding the factors associat- ed with waning immune response to mumps vaccine. Waning immunity to mumps is a public health concern and a major obstacle in the control and elimination of mumps disease. Outbreaks in the US and Europe pri- marily occur among highly vaccinated populations in their late teens and early twenties. These outbreaks are primarily due to the waning of mumps vaccine-induced immunity. Waning immunity to mumps is defined by decrease in antibody titer over time, decreased vaccine effectiveness in older subjects, and increased suscep- tibility to disease as time since vaccination increases; however, little information is available on the qualitative characteristics of mumps-specific memory B cells, and even less data is available regarding long-term T cell responses to mumps. Our goal is to study the first factor (decrease in immune responses over time). The es- tablishment of long-lasting immunity to mumps is likely to be controlled by multiple immunologic pathways, networks, and cell types acting in a coordinated and interactive manner. We propose to characterize B cell and CD4+ T cell responses to a 3rd dose of MMR. We will then examine how baseline characteristics (e.g., prior Ab response and sex) and genetic factors (mRNA and miRNA) influence those immune responses using a sys- tems-biology approach and mediation analysis. In order to accomplish this objective, we propose the following Specific Aims: 1) To conduct a systems biology study of early humoral (B cell) immune responses; 2) To con- duct a systems biology study of early CD4+ T helper responses; and 3) To characterize interrelationships be- tween T helper cell and humoral immune responses. These aims will allow us to comprehensively assess how? and to what degree?baseline immunity, the early primary response to a 3rd dose of MMR, and the es- tablishment of memory and return to homeostasis after MMR vaccination contribute to the observed clinical waning of mumps immunity. In turn, this knowledge can be utilized to inform mechanistic studies of mumps vaccine-induced immune persistence, develop more effective mumps vaccines and adjuvants in the future, and to identify possible early biomarkers to detect those at greatest risk of waning protection against mumps.